1. Field of the Invention
The present invention relates to protection devices for the eyes or other optical sensors and more particularly, to a device for protecting an eye or optical-sensor from intense light sources, such as a laser or atomic blast, which can have extremely rapid rise times and be of an unknown wavelength or of multiple wavelengths.
2. Description of the Prior Art
High powered light sources, and, in particular, lasers, are being used increasingly in industry and within the military for many applications. In the industrial community, lasers may pose an extreme threat to workers using them. This threat is usually controlled or neutralized quite easily because the laser wavelength is known and goggles or other protective apparatus with band reject filters for rejecting that particular wavelength can be used. However, even in the controlled industrial environment some processes or applications require the use of fixed multi-wavelength or variable wavelength (dye tunable) lasers, thus making eye and optical-sensor protection more difficult. In a military environment, lasers are used for targeting and rangefinding as well as in offensive applications. As a result, the eyes, and optical-sensors of all types, run an extreme risk of damage from laser light produced by "friendly" sources as well as enemy sources. Further, space reconnaissance satellites are another obvious target for laser attack.
In response to this threat, the Defense Advanced Research Projects Agency (DARPA) has initiated a program to develop methods for providing protection against laser threats. This program is currently focused on eight different approaches.
Briefly considering these approaches, one candidate involves the use of phase-transition switches. To explain, certain materials undergo drastic changes in the optical properties thereof which are associated with normal phase changes. For example, vanadium oxide is light transparent in the semi-metallic phase and highly reflective in the metallic phase. This phase transition can theoretically be induced by a slight temperature increase, such as would accompany a laser strike. Although this is an extremely promising approach to sensor protection, actual fabrication of acceptable devices has presented problems.
Self-induced gratings are also being considered. In some materials, the dielectric constant is a non-linear function of the intensity of an applied electric field and work is being done to find a material in which standing waves from a laser are able to induce sufficient dielectric change to cause the material to act as a diffraction grating and thus cause the incident energy to be scattered out of the optical path. Materials with sufficient non-linear behavior to provide protection of the type desired have not yet been identified.
Another approach is to use self-induced focusing. Non-linear optical materials can also be exploited by arranging optical paths that change focal distance as a function of light intensity. However, while this effect has been demonstrated, the effect produced by materials tested to date is not of sufficient strength to provide the protection desired.
Another approach involves scattering in a nonhomogeneous medium. Some materials change optical properties locally as a function of light intensity. For example, small spheres of a non-linear optical material can be suspended in a polymer of similar refractive index. As the light intensity increases, the dielectric constant of the spheres changes much more rapidly than the ambient, thus causing scattering. Although this approach is promising, the practical application thereof to the problem of laser light protection is limited by the lack of availability of suitable materials.
Coherence filters have also been considered and some theoretical approaches to producing a filter which can discriminate and reflect coherent light (laser light) have been proposed and demonstrated in a weak form. This approach could produce an ideal laser protection device if realized, but, at present, the technology is not available.
One way to circumvent the weak non-linear dielectric effect of existing materials is to bring the incoming energy to a focus and then apply one of the techniques listed above. For example, a unity-gain telescope could be used with a protective device located at a focal plane where the energy is concentrated. This approach can be enhanced or exanded with a microlens array. A microlens array is effectively an array of small unity-gain telescopes arranged as a goggle or protective filter. This approach also has some promise, but still depends on finding suitable non-linear optical materials or the like and has the additional drawback that the image quality is seriously degraded when presented as a series of overlapping images.
An old approach in laser protection is to provide an optical path with a sacrificial reflecting element (mirror) in the path. As energy exceeds a certain threshold, the reflecting element will "burn off," thus blocking the optical path. This approach has several disadvantages: the technology involved in the mirror design is critical (and as yet unsuccessful), the optical design is often by neccesity cumbersome in order to accommodate a reflecting surface, and once sacrified, the device is non-functional.
A technologically feasible or realizable approach to sensor protection is to substitute another sensor in the optical path. For example, a human can observe a scene via a television link and be perfectly safe. This approach certainly has advantages, with the primary one being that the technology is currently available and known to work. However, the initial sensor in the system, i.e., the vidicon camera in the example referred to is not protected at all and can be destroyed if exposed to strong laser light.
Modern laser weapons have extremely fast onset times (Q switch) and high power output per pulse. Such weapons can also emit energy at an unknown wavelength. These characteristics imply that conventional approaches to providing eye and sensor protection based on either fast, responsive switching or selective wavelength-dependent filtering are unlikely to be adequate. Moreover, the more esoteric approaches to protection discussed above are not as yet reliable enough, advanced enough or sophisticated enough to enable the use thereof in practical protective devices.
Patented devices of interest here include those disclosed in U.S. Pat. Nos. 3,245,315 (Marks et al); 4,842,400 (Klein); 4,264,154 (Petersen); 4,848,890 (Horn); 4,968,127 (Russell et al); and 4,978,208 (Hsu et al).
Briefly considering these patents, the Marks et al patent discloses eyeglasses or spectacles which are said to be capable of protecting the eyes of a wearer from damage by a blinding flash of light. A photocell controls the opening and closing of two electro-optic shutters mounted in the eyeglass frame. The shutters comprise conductive coatings or crystal plates which become darkened when the intensity of the light received by the photocells exceeds a safe level.
The Klein patent discloses eyeglasses which are designed to prevent the wearer from being blinded by excessive luminous intensity (produced, e.g., by a welding plasma). The eyeglasses include a pair of plates each having strips or zones thereon which can selectively be made opaque or transparent under the control of a scanning unit in the form of a shift register. The plate scanning frequency is preferably in excess of 16 Hz which, due to the persistence of vision, makes the scan invisible.
The Petersen patent discloses polarizing sunglasses including an arrangement for automatically controlling the transmission of light through the lenses of the glasses. An actuator incrementally rotates a polarized element in response to the brightness of the light received by a sensor through the glasses.
The Horn patent discloses an eye protection device wherein liquid crystal matrices are positioned over the eyes of a wearer and a sun-tracking photosensor determines the area of direct sunlight in the field of view of the wearer.
The Russell patent discloses eyeglasses including electronically controlled liquid crystal lens assemblies wherein the optical transmissivity thereof is automatically controlled to a level correlated to the intensity level of the ambient light as sensed by a photocell array.
The Hsu et al patent discloses eye protection goggles incorporating a pair of spatial light modulators comprising a photosensor diode and a photoemitting diode array. Each modulator has two semiconductive layers of opposite electrical polarities that are sandwiched together with layers of the same polarity (P or N) in electrical contact with each other.
Patents relating to goggles, eyeglasses, sunglasses and the like which include louvers, shutters or other movable parts for selectively blocking or modifying the light received by the eyes of a wearer include the following: U.S. Pat. Nos. 2,642,569 (Triebes et al); 2,773,411 (Schwede); 2,824,308 (Duncan); 3,689,136 (Atamian); 3,752,567 (Broadhurst); 4,386,832 (Nannini); 4,396,259 (Miller); 4,511,225 (Lipson); 4,595,262 (Ogle, deceased); 4,869,584 (Dion); and 4,953,231 (Burnett). Very briefly considering these patents, the Triebes et al patent discloses a sunguard for the eyes comprising glasses including a removable semi-opaque plate. The Schwede patent discloses glasses, including rotating optical lenses, for reducing glare. The Duncan patent discloses eyeglasses including a louvered screen. The Atamian patent discloses sunglasses with reversible shade portions. The Broadhurst patent discloses eyeglasses with interchangeable colored lenses. The Nannini patent discloses "adjustable luminous intensity" sunglasses including movable lenses. The Miller patent discloses "spectrum glasses" including a rotating color wheel. The Lipson patent discloses variable neutral density laser goggles including a rotatable polarized element. The Ogle patent discloses tunable birefringent safety goggles including a rotating polarizer. The Dion patent discloses louvered sunglasses. The Burnett patent discloses a shade attachment for eyeglasses including closeable slats.